Abstract: The study of subduction zone dynamics is important in plate tectonics and geodynamics. The Bismarck Sea, located in the southwestern Pacific plate convergence zone, is bordered by the Caroline Plate to the north, the Eurasian tectonic domain to the west, the Australian Plate to the south, and the Ontong Java Plateau to the east, making it one of the most tectonically active regions in the world. We investigated the spatial distribution of the regional stress field in the Bismarck Sea based on focal mechanism solutions and quantitatively analyzed the block motion patterns using multi-source GPS data, revealing systematically the stress field characteristics and dynamic mechanisms of the area. Results show that the focal mechanism solutions near New Guinea Island and the New Britain Trench are predominantly of reverse faulting type, dominated by the northeastward subduction of the Indo-Australian Plate along the New Britain Trench. The T-axis strikes in the NW-SE direction, while the P-axis extends in the NE-SW direction, resulting in a strong clockwise rotation of the South Bismarck Ocean Basin. The focal mechanism solutions near the West Melanesian Trench are also mainly in reverse faulting type, with the development of SW-dipping thrust faults. In the southwestern region adjacent to the trench, the T-axis exhibits a nearly NW-SE distribution, and the P-axis shows a NE-SW orientation. For the southeastern area of the trench, the T-axis is approximately E-W trending, indicating that the Pacific Plate has gradually become imbricated with the North Bismarck Ocean Basin at the West Melanesian Trench. The boundary zone between the Southern and Northern Bismarck Basins, under the background of oblique convergence between the Pacific and Indo-Australian plates, generally exhibits strike-slip focal mechanism solutions, forming a left-lateral strike-slip fault system. Its shear deformation characteristics are highly coupled with the regional NE-SW trending principal compressive stress field.